Output buffers, or drivers, are commonly used in digital circuits to provide a means for generating one of two voltage levels. Typically, the voltage levels correspond to logic levels such as zero and one, or, analogously, low and high. When the output buffer is implemented in metal oxide semiconductor ("MOS") process technology, the output driver circuit is commonly comprised of two transistors connected between ground and the circuit supply voltage, Vcc. The output signal is taken from a point on the connection between the two transistors called the output node.
The transistor connected between the output node and Vcc will "pull" the output node voltage high when switched on. The second transistor, connected between the output node and ground, pulls the output node low when turned on. Thus, the output node can be set to output a high voltage, Vcc, or a low voltage, ground.
An output buffer must usually output a large current, switch between voltage levels quickly and maintain a low level of electrical noise generation. A large output current may be necessary to meet the input requirements of another circuit which uses the signal from the output buffer. Fast switching times are essential in high-speed digital circuits and noise suppression is always desired, especially when the digital circuit is fabricated as a microcircuit on the substrate of a chip with a high density of components.
However, designing an output buffer with the above characteristics is difficult, since the characteristics are interdependent and an improvement of one characteristic is usually at the expense of another characteristic. For example, increasing the current output generally requires the use of larger components on the chip substrate. This creates larger parasitic capacitances which slow down the switching speed. Also, the faster that a signal is switched and the greater the current that is switched, the greater will be the noise generated by the output buffer. This increased noise can interfere with the functioning of other components. The trends toward denser packing of components on a substrate in a microcircuit makes noise reduction or suppression especially important in output buffer designs.
One form of unwanted noise generated by an output buffer is known as "ground bounce." This occurs when an output buffer switches its output node from the high voltage level to the low voltage level. During the switching, a transient ground current is generated which causes the ground line to oscillate momentarily. The magnitude of the ground bounce is larger when the voltage switching range increases or when the output current of the buffer is larger. Since the ground line, or ground plane, is shared by many devices either on a chip substrate or on a circuit board, the ground bounce, if large enough, could cause a chip or circuit to malfunction.
The problem of ground bounce becomes magnified when it is desired to operate several output buffers with a common ground plane. As an example, when several output buffers with a common ground plane happen to switch simultaneously, the resulting transient current generated in the ground plane will be on the order of tens of milliamps where the switching speed is on the order of nanoseconds. This can translate to a ground bounce voltage of two or more volts seen by other devices attached to the ground plane. This voltage may be above the threshold for a logic high level and would result in an error in the logic of the chip or circuit.
Presently, the problem of ground bounce is critical in the design of faster MOS devices. One way to minimize ground bounce is to prevent the high logic level from going too high. This reduces the voltage "swing" from a high to low output voltages and reduces the voltage "spike" caused by a voltage transition through a capacitive load which creates ground bounce.
For example, in a circuit where the supply voltage, Vcc, is 5 volts, a high logic level in the range of 2.5 to 3.5 volts may be desired. For various reasons, e.g., a loosely regulated power supply or power surges on the supply line, Vcc may increase briefly. Since an output buffer's logic high output level is derived from Vcc, a rise in Vcc would mean a rise in the high logic level being output by the buffer. Thus, when the output buffer attempts to swing from a high to a low transition (low being ground or zero volts), the high voltage level may be elevated above the 2.5 to 3.5 volt range requiring the output buffer to provide a larger voltage swing to achieve the high to low transition. This increased voltage swing results in a larger amplitude of ground bounce. Therefore, it is desired to control or regulate the voltage level of the output buffer so that the logic high output voltage will not be increased with an increase in Vcc.
A further problem in regulating or controlling the maximum output voltage for a high logic state in an output buffer is that the "rise time," or transition time in going from a low logic state to a high logic state, must be kept as short as possible. This means that the circuit which regulates or controls the output voltage must not degrade the rise time of the output buffer in switching between a low to a high logic state. A voltage control circuit will also be required to maintain a constant and sufficiently uniform logic high state over a period of time.